At 58, Bill Gates (Trades, Portfolio) is not only the richest man in the world, with a fortune that now exceeds $76 billion, but he may also be the most optimistic. In his view, the world is a giant operating system that just needs to be debugged. Gates' driving idea – the idea that animates his life, that guides his philanthropy, that keeps him late in his sleek book-lined office overlooking Lake Washington, outside Seattle – is the hacker's notion that the code for these problems can be rewritten, that errors can be fixed, that huge systems – whether it's Windows 8, global poverty or climate change – can be improved if you have the right tools and the right skills. The Bill & Melinda Gates Foundation, the philanthropic organization with a $36 billion endowment that he runs with his wife, is like a giant startup whose target market is human civilization.

Personally, Gates has very little Master of the Universe swagger and, given the scale of his wealth, his possessions are modest: three houses, one plane, no yachts. He wears loafers and khakis and V-neck sweaters. He often needs a haircut. His glasses haven't changed much in 40 years. For fun, he attends bridge tournaments.

But if his social ambitions are modest, his intellectual scope is mind-boggling: climate, energy, agriculture, infectious diseases and education reform, to name a few. He has former nuclear physicists helping cook up nutritional cookies to feed the developing world. A polio SWAT team has already spent $1.5 billion (and is committed to another $1.8 billion through 2018) to eradicate the virus. He's engineering better toilets and funding research into condoms made of carbon nanotubes.

It's a long way from the early days of the digital revolution, when Gates was almost a caricature of a greedy monopolist hell-bent on installing Windows on every computer in the galaxy ("The trouble with Bill," Steve Jobs once told me, "is that he wants to take a nickel for himself out of every dollar that passes through his hands"). But when Gates stepped down as Microsoft CEO in 2000, he found a way to transform his aggressive drive to conquer the desktop into an aggressive drive to conquer poverty and disease.

Now he's returning to Microsoft as a "technology adviser" to Satya Nadella, Microsoft's new CEO. "Satya has asked me to review the product plans and come in and help make some quick decisions and pick some new directions," Gates told me as we talked in his office on a rainy day a few weeks ago. He estimates­ that he'll devote a third of his time to Microsoft and two-thirds to his foundation and other work. But the Microsoft of today is nothing like the world-dominating behemoth of the Nineties. The company remained shackled to the desktop for too long, while competitors – namely, Apple and Google – moved on to phones and tablets. And instead of talking in visionary terms about the company's future, Gates talks of challenges­ that sound almost mundane for a man of his ambitions, like reinventing Windows and Office for the era of cloud computing. But in some ways, that's not unexpected: Unlike, say, Jobs, who returned to Apple with a religious zeal, Gates clearly has bigger things on his mind than figuring out how to make spreadsheets workable in the cloud.

When you started Microsoft, you had a crazy-sounding idea that someday there would be a computer on every desktop. Now, as you return to Microsoft 40 years later, we have computers not just on our desktops, but in our pockets – and everywhere else. What is the biggest surprise to you in the way this has all played out?
Well, it's pretty amazing to go from a world where computers were unheard of and very complex to where they're a tool of everyday life. That was the dream that I wanted to make come true, and in a large part it's unfolded as I'd expected. You can argue about advertising business models or which networking protocol would catch on or which screen sizes would be used for which things. There are less robots now than I would have guessed. Vision and speech have come a little later than I had guessed. But these are things that will probably emerge within five years, and certainly within 10 years.

If there's a deal that symbolizes where Silicon Valley is today, it's Facebook's $19 billion acquisition of WhatsApp. What does that say about the economics of Silicon Valley right now?
It means that Mark Zuckerberg wants Facebook to be the next Facebook. Mark has the credibility to say, "I'm going to spend $19 billion to buy something that has essentially no revenue model." I think his aggressiveness is wise – although the price is higher than I would have expected. It shows that user bases are extremely valuable. It's software; it can morph into a broad set of things – once you're set up communicating with somebody, you're not just going to do text. You're going to do photos, you're going to share documents, you're going to play games together.

Apparently, Google was looking at it.
Yeah, yeah. Microsoft would have been willing to buy it, too. . . . I don't know for $19 billion, but the company's extremely valuable.

You mentioned Mark Zuckerberg. When you look at what he's done, do you see some of yourself in him?
Oh, sure. We're both Harvard dropouts, we both had strong, stubborn views of what software could do. I give him more credit for shaping the user interface of his product. He's more of a product manager than I was. I'm more of a coder, down in the bowels and the architecture, than he is. But, you know, that's not that major of a difference. I start with architecture, and Mark starts with products, and Steve Jobs started with aesthetics.

What are the implications of the transition to mobile and the cloud for Microsoft?
Office and the other Microsoft assets that we built in the Nineties and kept tuning up have lasted a long time. Now, they need more than a tuneup. But that's pretty exciting for the people inside who say, "We need to take a little risk and do some new stuff" – Google, which is a very strong company across a huge number of things right now.

Yeah, they were sort of born in the cloud.
The fact is, search generates a lot of money. And when you have a lot of money, it allows you to go down a lot of dead ends. We had that luxury at Microsoft in the Nineties. You can pursue things that are way out there. We did massive interactive­TV stuff, we did digital-wallet stuff. A lot of it was ahead of its time, but we could afford it.

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